By now you've identified all the subjects that the database will track
and defined the table structures that will represent those subjects.
Furthermore, you've put the structures through a screening process to
control their makeup and quality. In this next stage of the database-design
process, you'll begin the task of assigning keys to each table.
You'll soon learn that there are different types of keys, and each plays a
particular role within the database structure. All but one key is assigned
during this stage; you'll assign the remaining key later (in Chapter 10) as
you establish relationships between tables.

Why Keys Are Important

Keys are crucial to a table structure for the following reasons:

They ensure that each record in a table is precisely identified.
As you already know, a table represents a singular collection of similar objects
or events. (For example, a CLASSES table represents a collection of
classes, not just a single class.) The complete set of records within the table
constitutes the collection, and each record represents a unique instance of the
table's subject within that collection. You must have some means of
accurately identifying each instance, and a key is the device that allows you to
do so.

They help establish and enforce various types of integrity. Keys
are a major component of table-level integrity and relationship-level integrity.
For instance, they enable you to ensure that a table has unique records and that
the fields you use to establish a relationship between a pair of tables always
contain matching values.

They serve to establish table relationships. As you'll learn
in Chapter 10, you'll use keys to establish a relationship between a pair
of tables.

Always make certain that you define the appropriate keys for each table.
Doing so will help you guarantee that the table structures are sound, that
redundant data within each table is minimal, and that the relationships between
tables are solid.